Hi Batfish,
For twin engine sound - you hook up both throttle channels to the sound card using 2 connections
You can specify for each engine - the startup sound, idle sound, engine run sound ( and more) - but these are likely to be the same wav files if both engines are the same.
So you will hear each engine start individually assuming you are using TX sticks / 2 RX throttle channels.
If you leave one engine ahead and put one engine into reverse you will hear that engine sound slow down and then speed up to the reverse speed selected - whilst the other engine keeps doing it's thing unaltered...
You can specify an additional sound that plays as the main engine goes into reverse - could be a voice announcement/ bells sounding etc
There is lots more functionality on this sound/light card - it can switch things on / off - synch sound triggers to events - on off
You can write 2 "sequences" to flash an LED or move a servo - this could be for a servo go to 30 degrees and stay there for 5 seconds, move to 90 degrees wait for x seconds
If you use this sequence for LED flashing you can see how you can determine how long the LED is lit and how long the gap is until it is relit - hence the ability to get a LED to flash in a loop eg
Short Flash - gap - Long Flash - gap - Short flash - gap - Short flash - repeat
Regarding speaker size & box - I use these also sold by Beier - Speaker and then box below
https://www.beier-electronic.de/modellbau/produkte/ls-4r-25w-90/ls-4r-25w-90.phpYou can see this speaker / speaker box in this video
https://www.youtube.com/watch?v=dFFTR6rjUFMYou may well be able to get the speaker cheaper elsewhere
If you do decide on this option - it is worth buying the Data cable that allows you to control the card directly from your PC - when you are configuring it it will save you lots of time not having to eject/insert the micro sd card
And lastly - another demo board of using this for a truck - all sound / light / servo controlled by USM RC2( with some switching help from their speed controller) - check out the sound synchronising - indicator to sound and windscreen wiper servo/sound
https://www.youtube.com/watch?v=s4uzjzyIL3QRegards
C-3PO